The invention relates to pulverizers of the type that are used in pulverized coal fired steam generating systems. More particularly, the invention relates to apparatus that scrapes accumulated substances from the underbowl of a bowl mill.
Pulverized coal firing is favored over other methods of burning coal because pulverized coal burns like gas and, therefore, fires are easily lighted and controlled.
Pulverizers, also referred to as mills, are used to grind or comminute the fuel. The present invention has primary application to bowl pulverizers or mills. Although the present invention will be described with respect a bowl type ring roll pulverizer. Those skilled in the art will recognize that the invention may be used in other bowl type mills as well.
In the operation of a pulverizer coal enters the center of the pulverizer and through a center feed pipe. The coal is pulled onto a rotating bowl which has a replaceable wear surface. Centrifugal force causes the coal to move outward from the center and under three journal assemblies, where it is crushed by large rolls. The partially pulverized coal passes over the rim of the bowl and is it entrained by a rising hot air stream. Pyrites and tramp iron that enter the mill with the coal follow the same path as the coal until they pass over the rim of the bowl. Because these materials are more dense than coal, they are not carried any further upward by the air stream and fall into an underbowl. In addition to pyrites and tramp iron there may be other heavy foreign materials that may be found in the coal that will also fall into the underbowl.
These materials are both very abrasive and erosive. Removal of these materials from the underbowl requires the use of a scraper to mechanically push the materials toward an outlet chute. The abrasion and errosion caused by these materials is prejudicial to the service life of the scraper apparatus.
The hostile environment in which the scraper mechanism must operate will be more apparent by considering both the nature of the materials that pass through the pulverizer as well as the quality of material that flows through the pulverizer. A typical pulverizer has a capacity of up to 200,000 pounds of coal per hour.
Prior art scrapers have included a generally planar scraping element carried on a vertical axis. Centrifugal forces urge the scraping element to a generally radial position. A central body carries the vertical axes of the scraping elements. As the central body rotates the scraping elements are urged by centrifugal forces to generally radial positions about the underbowl. If a large object falls into the underbowl the scraping element can move from the radial position upon impact with the large object and then return to the radial position after passing the large object.
It is believed that the vertical axis of the prior art scraper inherently positions a part of the bearing surface in close proximity to severely hostile abrasive and erosive environment. More particularly, the lower extremity of the vertical axis inherently must be very close to a hostile environment that it is prejudicial to the bearing.
Another concern with the prior art scraper assembly is that it requires installation of a complete assembly to repair worn parts.
In some cases wear of the vertical axis and journal bearing cooperating with the axis may allow the generally planar scraper element to droop so that the radial extremity of scraper element contacts the surface of the bowl and causes wear. Such constructions have been satisfactory for many installations. However, in some installations the materials found within a particular coal may be more abrasive and more erosive than in other coals from other parts of the world.
Another concern is that heavy assemblies are more difficult to install.